Control panels for air conditioning operations in vehicles are comprised with light-emitting diodes to make letters and symbols indicating functions allotted to switches easily readable and to display operating conditions of the switches. There are known brightness adjusting functions such as reducing the brightness of light-emitting diodes at night times. For realizing such brightness adjusting functions, PWM (pulse width modulation) control is generally employed which requires a small number of parts and involves low costs.
Touch switches of electrostatic capacitance type are widely used in these years due to reasons that they can realize simple designs free of unevenness and that they are free of malfunctions caused through wear of switch contacts.
Touch switches of electrostatic capacitance type include electrodes that are disposed proximate of operating portions that are subject to touch operations. Touch operations are detected based on changes in electrostatic capacitances in accordance with a conductive body such as a human body approaching these electrodes.
More particularly, detected values of sizes that are in accordance with electrostatic capacitances of the electrodes are obtained at constant time intervals by a detecting circuit. When a difference between the latest detected value and a reference value is less than a prescribed difference value Δa, it is judged that no touch operations have been made, and the reference value is updated to the latest detected value. When a difference between the latest detected value and a reference value is not less than a prescribed difference value Δa, it is judged that a touch operation has been made, and the reference value is not updated.
FIG. 9 is a time chart showing one example of changes in detected values.
In this case, while reference values are replaced by the latest detected values up to time point T11, the reference value is constant at Ra since the detected values Ra are constant.
At time point T12, since the difference between the latest detected value and the reference value is not less than the prescribed difference value Δa, the reference value is not updated but remains Ra. Accordingly, since all of the differences between the latest detected values and the reference values will be not less than the prescribed difference value Δa until to the next time point T13, the reference value is not updated but remains Ra.
While the reference value is updated at time point T14 since the difference between the latest detected value and the reference value Ra is less than the prescribed difference value Δa, the reference value remains Ra since the latest detected value is Ra.
In this manner, the reference remains almost constant at Ra, and touch operations are detected during period P11 between time point T12 and time point T14.